The present invention relates to the technical field of organic-inorganic composite or hybrid materials, especially to the incorporation of inorganic nanoparticles (such as e.g. metal oxide nanoparticles) in organic polymers (such as e.g. latexes).
Especially, the present invention relates to a method for producing a nanocomposite dispersion comprising composite particles of inorganic nanoparticles and organic polymers in a continuous phase, especially in a dispersion medium, and to the nanocomposite dispersion thus obtained as well as to composite particles of inorganic nanoparticles and organic polymers obtainable from this nanocomposite dispersions.
Furthermore, the present invention refers to the use of the nanocomposite dispersions of the present invention or of the composite particles of the present invention, respectively, in plastics and polymeric compositions, in coatings, in paints, in lacquers and adhesives, especially in applications for wood-based materials or substrates.
Further, the present invention refers to plastics and polymeric compositions, coatings, paints, lacquers and adhesives, which comprise the nanocomposite dispersions of the present invention or the composite particles of the present invention, respectively, especially in applications for wood-based materials or substrates.
Finally, the present invention refers also to additive compositions, especially for use in plastics and polymeric compositions, coatings, paints, lacquers and adhesives, which additive compositions comprise nanocomposite dispersions of the present invention or composite particles of the present invention, respectively.
Incorporation of inorganic nanoparticles, especially metal oxide nanoparticles, in polymer matrices or polymer particles has become of particular interest due to the wide range of applications of the resulting composite or hybrid materials, such as e.g. in coatings, adhesives, medicine, cosmetics etc., for instance, also in the field of wood-based materials. With the incorporation of inorganic nanoparticles into polymeric matrices, improved properties of the composite or hybrid materials may result.
Wood-based materials find a broad range of applications, especially in civil engineering, e.g. as construction material, in furniture industry and the like.
The term “wood-based materials” as used herein denotes, beside solid wood, also wooden materials such as fiber-, chip-, and particleboard materials. The wood-based material can be any type and form of wood based material such as chips, fibers, sheets, laminas, veneers, pieces etc. The wood-based materials or products may, for instance, be a laminated or veneered material. The wood-based materials or products may, for instance, also be a composite product such as e.g. a particle board, fiber board (such as MDF), chip board or oriented strand board. The wood-based materials or products may, for instance, be a laminated or veneered material, such as laminated flooring, veneered flooring, a veneered furniture material, plywood, a wall panel, a roofing panel, a laminated beam, or a composite product.
Wood has an intrinsic potential to fulfill the criteria for being a competitive and sustainable engineering material, i.e. it is a renewable resource available in vast quantities and formed as a natural composite with an extraordinary high strength-to-weight ratio. For outdoor use it is, however, necessary to enhance the performance and long-term durability of wood-based materials.
Especially, wood is an excellent building material with a high strength/density ratio, it is a renewable resource and has been used successfully for centuries. When wood is used as a building material, it is also desirable to expose the beautiful surface structure of the wood material, thus clear coatings is the ideal choice of surface treatment for wood products. However, protection of lignin and other carbohydrates during weathering of wood is one of the main challenges in outdoor applications of clear coatings on various wood products.
One particular problematic area and drawback for the use of wood as an engineering material outdoor is its high sensitivity for UV degradation. It is common knowledge that wood is affected by light, both by color changes and by degradation of the surface. Particularly, during outdoor exposure, the effects are rapidly noticeable on untreated wood, which becomes grey and more loosely bound raised fibers become visible at the surface. This is mainly because the surface lignin (which represents ca. 30% of the wood weight) is degraded into smaller molecular fragments that can be washed out by rain.
Traditionally, UV protection of wood has been performed through hiding the sensitive lignin under pigments, through applying a tinted wood coating, which efficiently hinders the UV light to reach the lignin. To date, this method has been widely accepted in the Scandinavian and North American countries, but is not fully accepted in other countries, e.g. in southern Europe and Japan.
Moreover, a recent trend within architecture is to include clear-coated wood as a significant part of the exterior design, thereby taking advantage of the appealing aesthetic properties of the wood material.
Furthermore, it is also important to develop new improved clear coating systems from a legal point of view. In some European countries, for example in Germany, there is a need for a warranty from the coating companies that the coating will last for a certain period. With improved clear coatings it would be easier for the coating companies to give these warranties.
To protect wood products effectively during weathering, the high energetic portion of the sunlight spectrum, i.e. UV-VIS (250 to 440 nm), should be cut off or filtered before it reaches the wood surface. Due to tighter regulations and environmental concerns in the coating sectors, water-borne clear coatings are more appreciated compared to the solvent-based ones.
Traditionally, organic UV absorbers are used in clear coatings for wood applications, however, these substances degrade rapidly upon outdoor weathering. As a result of this, the coating not only loses its intended UV-protection function very quickly, it also contributes to volatile organic compounds (VOCs) emission to the environment. Current clear coating systems with organic UV absorbers need to be replaced or repainted after approximately 2 years.
Thus, if superior UV-absorbing systems could be identified for water-based clear coatings, the service-life of the wood could be extended and the environmental impact and the cost for maintenance and for wood replacement would be decreased.
Although several inorganic fillers, such as TiO2, ZnO, Fe2O3 and CeO2, have been extensively studied as potential candidates in various clear coating formulations, little progress has been made so far, due to the difficulty of accessing transparent inorganic fillers that are compatible with water-based clear coating formulations. Therefore, in wood coating industry, there is an urgent need to improve transparency, color stability and durability of transparent wood coatings for outdoor applications.
Functional nanoparticles can also be introduced into water-based wood adhesives in order to improve the properties of wood-adhesive joints. The most commonly used adhesives in wood applications are the thermoset formaldehyde-based adhesives (urea-formaldehyde UF, melamine-formaldehyde MF, phenyl-formaldehyde PF, resorcinol-formaldehyde RF). However, due to formaldehyde emission problems, these adhesives should be phased out in the future.
Other alternatives are thermoplastic adhesives from petroleum sources or biobased adhesives. For example, water-based polyvinyl acetate adhesives are used for gluing of wood. This adhesive has some limitations due to the thermoplastic character. It is sensitive to both moisture and heat and tend to creep under load. This limits its application to non-structural uses only, and it is not used at elevated temperatures.
However, if the mechanical properties of polyvinyl acetate or of biobased thermoplastic adhesives could be improved, these adhesives could be used for a wider range of wood applications in the future.
For example, if the creep were reduced for these new adhesives, they might replace some formaldehyde-based adhesives in load bearing applications.
Incorporation of nanoparticles in water-based adhesives can also increase the cohesive strength of the adhesive without decreasing the adhesive properties of the adhesive. However, it is crucial that the nanoparticles are well dispersed in the polymer matrix and that the nanoparticles are compatible with the adhesive polymer. However, with conventional systems, only low compatibilities between the polymer matrix, on the one hand, and the nanoparticles, on the other hand, could be reached so far, resulting in non-homogeneous systems or in systems with low durability or life-time. In addition, the price of the nanoparticles used in this application so far is too high.
In the state of the art, several approaches have been made in order to increase the compatibility of such nanoparticles; however, so far these results are not very sufficient (see e.g. US 2002/0055581 A1, US 2002/0086908 A1, EP 1 471 108 A1 etc.).